Lung tissue resistance, Rtis, has been identified as a large and labile component of lung resistance. While the connective tissue matrix and the surface film contribute to Rtis, this project focuses in particular upon the contribution of intraparenchymal contractile tissues. We propose to identify the specific mechanism of energy dissipation accounting for Rtis and its changes with contractile stimulation. We have shown previously that tissue resistance can be decomposed into the product of tissue stiffness E and tissue hysteresivity eta, the latter of which is found empirically to be a highly conserved tissue property. Our central hypothesis is that, following the onset of sustained stimulation of the intraparenchymal contractile apparatus, recruitment of numbers of cross bridges attached accounts for the development of tissue stiffness while decrease in their apparent cross bridge cycling rate accounts for the evolution in time of tissue hysteresivity (as rapidly cycling cross bridges convert to slowly cycling latch bridges). Hence, the hypothesis holds that dynamic changes in Rtis attributable to contractile tissues can be interpreted mechanistically as the product of changes of cross bridge numbers aid their apparent cycling rates. These studies will investigate trachealis, which serves as a relatively homogeneous proxy for the kinetics of the intraparenchymal contractile element. We plan, first, to test a series of mechanistic predictions, the most important of which are that the time course of eta follows approximately that of unloaded velocity of shortening, a mechanical index of cross bridge cycling rate, and that the time course of Rtis follows closely that of actin-activated myosin ATPase measured by NADH fluorescence, a biochemical index of the product of cross bridge numbers and their cycling rates. Second, because the central hypothesis suggests a causal relationship between cross bridge kinetics, Rtis and myosin ATPase, we will test that relationship by modulating cross bridge kinetics in three independent ways. Third, we will develop a mechanistic quantitative theory linking macroscopic mechanical responses to underlying molecular kinetics at the level of the cross bridge, taking into account multiple bonding states. Fourth, we will evaluate how inflammatory processes modulate this mechanism and, fifth, we will evaluate the heterogeneity of the contractile response within the intraparenchymal contractile machinery. In pilot studies, temporal changes in unloaded velocity of shortening and rates of myosin ATP utilization have been measured and are shown to be closely related in time to changes of eta and eta-E respectively. Taken together, these data suggest that the actomyosin detachment event (in contrast to the classical viscous stress) seems to be of major importance in the change of Rtis with activation of the intraparenchymal contractile apparatus.